FORESHOCKS AND AFTERSHOCKS. 355 



Chapter VI. 

 FORESHOCKS AND AFTERSHOCKS. 



1. — Foreshocks. 



"When writing my preliminary report of this earthquake I had no 

 evidence at my disposal in favour of any foreshocks having been .felt in 

 the disturbed area. Since then, the detailed sifting of the earthquake 

 forms has revealed distinct evidences for a few premonitory shocks 

 that may be considered to be of the nature of foreshocks or warnings 

 of the big event. It is according to common knowledge that many 

 destructive earthquakes are preceded by a few such premonitory 

 shocks. Mr. Davidson writes : * " For a few hours or days before- 

 hand weak shocks and tremors are felt or a rumbling is heard within 

 the future meizoseismal area. In the 1891 earthquake in Japan the 

 previous two years was characterised by a greater frequency of earth- 

 quakes in this area. Even the future fault-scarp was mapped out by 

 them." 



So far as is at present known, there is no means, of distinguishing 

 such foreshocks from ordinary small shocks which may visit an area 

 without being succeeded by a great shock. Consequently their value 

 as actual warnings of a coming catastrophe is at present nil. This 

 may, however, be merely owing to our imperfect analysis of such 

 shocks. With a more perfect system of registration it may be that 

 in the future it will be possible to distinguish these warning tremors 

 from mere small local disturbances. This would be the more likely to 

 come about when seismographs are more plentifully distributed than 

 they are at present over known earthquake regions. 



In the brief list which follows it will be seen that the foreshocks 

 recorded (probably only a fraction of those which actually occurred) 



1 Hecent earthquakes by C. Davidson. 



2 a 



